A number of circuits have evolved which take advantage of the ability of field effect transistors to store charge and thus serve as memory cells. Such cells may be either dynamic or static in nature. The dynamic cells may employ only a single field effect transistor and the static cells may be arranged in a flip-flop configuration, as is well known. Each of these types of cells may be referred to as volatile cells since information stored in these cells is lost when the power supply voltage applied to the memory is lost or down. In instances where stored volatile information must be retained, an alternate power supply, such as a battery system, must be coupled to the memory for use in the event of failure of the main power supply.
Known devices for producing variable threshold voltages, such as field effect transistors having metal-nitride-oxide-silicon (MNOS) and field effect transistors which include a floating gate are capable of storing information in a non-volatile manner for long periods of time. By incorporating such non-volatile devices into memory cells, there has been provided normally operating volatile cells which do not require a backup or alternate power supply for preserving information when power interruption or failure occurs in the main power supply.
Non-Volatile memory cells which use the non-volatile MNOS devices are capable of retaining information stored volatilely in a cell but these devices require high voltage pulses for writing and erasing the information, they are slow and they require rather complex processes for their fabrication. Examples of non-volatile MNOS semiconductor memory cells are taught in U.S. Pat. Nos. 3,676,717, filed Nov. 2, 1970, 4,095,281, filed Mar. 4, 1976, 4,103,348, filed Aug. 29, 1977 and 4,122,541, filed Aug. 25, 1976.
Known non-volatile memory cells which use conventionally arranged floating gate devices are also capable of preserving information stored volatilely in a cell but these devices likewise require high voltage pulses for writing and erasing the information, they are slow and require high currents, approximately one milliampere per device, to write. Examples of known non-volatile semiconductor memory cells having incorporated therein volatile storage and using floating gates are taught in U.S. Pat. Nos. 4,128,773, filed Nov. 7, 1977 and 4,207,615, filed Nov. 17, 1978. These non-volatile floating gate devices generally operate with a tunneling mechanism which requires charge to be drawn from and directed into the semiconductor substrate through a very thin insulating layer.
In commonly assigned U.S. patent application Ser. No. 153,359, filed on May 27, 1980 by H. N. Kotecha, now U.S. Pat. No. 4,334,292, there is disclosed an electrically erasable programmable read only memory which utilizes a four port or terminal device having a floating gate with two control gates coupled to the floating gate. An enhanced conduction insulator arranged as a dual electron injector structure is disposed between the floating gate and one of the two control gates to charge and discharge the floating gate. Commonly assigned U.S. patent application Ser. No. 160,530 filed on June 18, 1980 by H. N. Kotecha and F. W. Wiedman, now U.S. Pat. No. 4,336,603, discloses a three port or terminal electrically erasable programmable read only memory which utilizes the enhanced conduction insulator. A detailed discussion of enhanced conduction insulators may be found in an article entitled "High Current Injection Into SiO.sub.2 from Si rich SiO.sub.2 Films and Experimental Applications" by D. J. DiMaria and D. W. Dong, Journal of Applied Physics 51(5), May 1980, pp. 2722-2735 and a basic memory cell which utilizes the dual electron injector structure is taught in an article entitled "Electrically-Alterable Memory Using A Dual Electron Injector Structure" by D. J. DiMaria, K. M. DeMeyer and D. W. Dong, IEEE Electron Device Letters, Vol. EDL-1, No. 9, September 1980, pp. 179-181.